This invention is directed to the removal of ionic silver from solutions using halogen impregnated activated carbon. Said solutions are usually aqueous; however, non-aqueous solutions containing silver metal complexes are also within the scope of the instant invention.
This invention is especially directed to the removal of ionic silver from photographic processing solutions such as, developers, fixers, bleach fixers and washwaters by contacting said solutions with a quantity of halogen impregnated activated carbon.
The term "ionic silver" as used herein, is defined as including both the cationic form of silver, Ag.sup.+, the anionic silver thiosulfate complex, [Ag(S.sub.2 O.sub.3).sub.2 ].sup.-3, and other soluble silver metal complexes.
Activated carbon has long been used as an adsorbent for removing metal ions from solutions. Carbon has been used in gold mining operations for the adsorption of Au(CN).sub.2.sup.-, gold-cyanide since about 1880. The activated carbon is generally believed to act as a reducing agent, for example, adding an electron to the cationic metal (M.sup.+) to form the elemental metal (M.degree.). For example, the red-ox potential for the reduction of silver (Ag.sup.+) is high, E.degree.=0.8 V and a weak reducing agent is sufficient for carrying out the reaction. The elemental silver (Ag.degree.) is then loosely bound to the activated carbon surface.
Darrah, in U.S. Pat. No. 4,056,261 describes the use of activated carbon, dilute cyanide and dilute caustic for the recovery of both gold and silver from mine-run dumps or low-grade crushed ores.
The silver complex present in most photographic processing solutions is known to exist as a stable, soluble silver-thiosulfate complex. The red-ox potential for the reduction of this complex to elemental silver is quite low, E.degree.=0.01 V. Because of the low red-ox potential a strong reducing agent is necessary to carry out the reduction reaction. Standard activated carbon is known to be a weak reducing agent and is ineffective for carrying out the reduction reaction. Due to this inability of standard activated carbon to reduce the complex the carbon has essentially no affinity for adsorbing the silver-thiosulfate complex.
Generally, the ionic silver present in photographic processing solutions is recovered by electrolytic methods. Generally, electrolytic recovery of ionic silver is useful only for ionic silver concentrations greater than 500 ppm. Typical electrolytic processes are described in U.S. Pat. Nos. 4,166,781; 4,111,766; and 4,026,784.
The disadvantages of the electrolytic method of silver removal from photographic solutions include; reducing the ionic silver concentration to below 500 ppm is difficult, capital expenditure for an electrolytic system is high, the system requires continuous monitoring and in addition to silver collecting at the cathode, the thiosulfate anion can be reduced to sulfide ion which immediately reacts with silver to form silver sulfide. This silver sulfide precipitate reduces the purity of the recovered silver.
Piret et al., in U.S. Pat. No. 4,131,451 describe the recovery of gold and silver from a chloride solution of these and other metals with the use of activated carbon and an aqueous iodide solution. The gold is immediately adsorbed by the carbon and iodide causes precipitation of silver compounds that are then separated from the gold-laden carbon. Critical in this process are the parameters of pH, temperature and oxidation capacity of the chloride solution.
Impregnated activated carbons are useful for adsorbing compounds that are either poorly adsorbed or not adsorbed at all by standard activated carbon. Carbon has been impregnated with NaOH (U.S. Pat. No. 4,072,479), silver (U.S. Pat. No. 3,294,572), tertiary amines with either bromine or iodine (U.S. Pat. No. 4,040,802), iodine alone (U.S. Pat. No. 4,075,282), and sulfur radicals (Japanese Kokai Koho 78,040,065).